feat(select): match options by expression other than object identity

Extend ng-options with a new clause, "track by [trackByExpression]", which can be used when
working with objects.  The `trackByExpression` should uniquely identify select options objects.
This solves the problem of previously having to match ng-options objects by identity.
You can now write: `ng-options="obj as obj.name for obj in objects track by obj.id"`
The "track by" expression will be used when checking for equality of objects.

Examples:
<select
    ng-model="user.favMovieStub"
    ng-options="movie as movie.name for movie in movies track by movie.id">
</select>

scope: {
  user: { name: 'Test user', favMovieStub: { id: 1, name: 'Starwars' } }
  movies: [{ id: 1, name: 'Starwars', rating: 5, ... }, { id: 13, ... }]
}

The select input will match user favMovieStub to the first movie in the movies array, and show
"Star Wars" as the selected item.

test/ng/directive/selectSpec.js

@@ -495,7 +495,7 @@ describe('select', function() {
       expect(function() {
         compile('<select ng-model="selected" ng-options="i dont parse"></select>');
       }).toThrow("Expected ngOptions in form of '_select_ (as _label_)? for (_key_,)?_value_ in" +
-                 " _collection_' but got 'i dont parse'.");
+                 " _collection_ (track by _expr_)?' but got 'i dont parse'.");
     });
 
 
@@ -753,6 +753,37 @@ describe('select', function() {
       });
 
 
+      it('should bind to scope value and track/identify objects', function() {
+        createSelect({
+          'ng-model': 'selected',
+          'ng-options': 'item as item.name for item in values track by item.id'
+        });
+
+        scope.$apply(function() {
+          scope.values = [{id: 1, name: 'first'},
+                          {id: 2, name: 'second'},
+                          {id: 3, name: 'third'},
+                          {id: 4, name: 'forth'}];
+          scope.selected = {id: 2};
+        });
+
+        expect(element.val()).toEqual('2');
+
+        var first = jqLite(element.find('option')[0]);
+        expect(first.text()).toEqual('first');
+        expect(first.attr('value')).toEqual('1');
+        var forth = jqLite(element.find('option')[3]);
+        expect(forth.text()).toEqual('forth');
+        expect(forth.attr('value')).toEqual('4');
+
+        scope.$apply(function() {
+          scope.selected = scope.values[3];
+        });
+
+        expect(element.val()).toEqual('4');
+      });
+
+
       it('should bind to scope value through experession', function() {
         createSelect({
           'ng-model': 'selected',